1. Field of the Invention
This invention relates to a switch designed to open and close under the control of a device such as an electromagnet and to open automatically in the event of overload current flow through the switch.
2. Description of the Prior Art
Switches such as contactors are already known in which the moving contact is directly connected to the armature of an electromagnet which controls the displacement of the moving contact against the action of restoring means, between a closed position and an open position of the contacts.
In some of these known contactors, means are also provided for initiating automatic opening of the contacts independently of the action of the electromagnet when the intensity of the current flow through the contactor oversteps a predetermined threshold value. These means can consist of a magnetic component which concentrates the field within a current loop in order to increase the intensity of the repulsion forces exerted on the moving contact. Automatic opening of the contacts is thus ensured when the current intensity exceeds a predetermined value which is usually of the order of thirty to forty times the rated value.
This type of switch thus has two different modes of operation as follows:
a contactor mode in which opening of the contacts is controlled solely by the electromagnet; PA1 a mode of operation in the event of occurrence of any fault condition in which opening of the contacts takes place automatically at the time of appearance of short-circuit currents, independently of the action of the electromagnet, even though the electromagnet is located in an active position in which the contacts should be closed.
A device of this type is described for example in French patent application No. 80 120 86 filed on May 30th, 1980 in the name of the present Applicant.
The device described in this patent application comprises a plurality of switches of the type consisting of a rigid contact bridge in cooperating relation with stationary contacts carried respectively by conductors which impart repulsion forces to the contact bridge. In addition, the central region of the bridge cooperates with a magnetic structure for subjecting the bridge to forces which compensate for the contact pressure below a predetermined threshold value. Above this value, the bridge is separated very rapidly from the stationary contacts.
This known device is so designed as to initiate the operation of the aforementioned magnetic structure only in order to produce an additional contact pressure force which is directly opposed to the electrodynamic repulsion forces.
This is subject to a disadvantage insofar as the speed of separation of the contacts (which is necessary when a short-circuit occurs) suffers from a balance of forces which is reduced by reason of the opposite directions in which the aforementioned additional force and the repulsion forces are exerted whereas it would be desirable to utilize all the forces which are present for the purpose of accelerating the speed of opening of the contact bridge and thus limiting the intensity of the short-circuit currents even more effectively.